A move that Vachier-Lagrave had played himself.
9. a2a3 White doesn't mind playing slowly to prevent the b5-b4 advance. The plan is not as brutal as usual in the 6.Bc4 line, when White usually plays f4-f5 to open the bishop's diagonal. Here the intentions are different. 9... e8g8  10. d1e2  10... c8b7  11. f2f3 White plans the traditional attack on the kingside with the advance of the pawns with g4-g5-h4-h5 and so on.